Propellant grain restriction



APU] 4, 1961 H. 1. MAHoN ET AL PROPELLANT GRAIN RESTRICTION Y Filed Aug. 11, 1950 f wg/(41( 4 wif y if if i i l l I IN VEN TORS HE/W? Y MAI-[0N BY MEEW YN C GIL L A MJ 7W A 7' TORNE Y 2-,977,ss4 PRoPELLAN'r GRAIN nnsrnlcrroN Henry I. Mahon, Honolulu, Hawaii, and Merwyn C. Gill, Los Angeles, Calif., assignors,` by mesne assignments, to Aerojet-General Corporation, Cincinnati, Ohio, a corporation of Ohio Filed Aug. 11, 195o, ser. No. 178,816 16 claims. (C1. 1oz-'98) This invention relates to propellants of the solid type used in rocket motors yand in particular to solid propellants made by polymerizing oxidizer-resin mixtures.

The principal object of this invention is to provide a liner -or restriction which is capable of effectively bonding itself to the walls of lsuch a resin-oxidizer propellant charge, thereby preventing unwanted burning on the lined surfaces when the grain is exposed to iiames and hot gases released during burning.

It: has been a common practice to form a solid propellaut charge in the form of a stick or grain and to insert such a charge, after curing, into the chamber of a rocket motor. v

When burning occurs on most or all surfaces, the grain is said to be burning unrestrictedly. Such unrestricted burning often generates indeterminately high pressures, and unless the motor is designed to withstand excessivey pressure the chamber may explode. To avoid unrestricted and nonuniform burning of a propellant grain, it has been the practice to line the surfaces of the grain on which no burning is desired with a slower burning, or nonburning material. The liner should adhere closely to the walls of the-propellant charge, thereby preventing the flames and hot gases generated during combustion from reaching and igniting the surfaces o-f the propellant grain vwhich are covered by the liner. v

stableoxidizers such as the inorganic nitrates, permaganat'esiand chromates, may be used. These substances have been more completely disclosed in the copending patent application of Roy Roberts, Serial No. 109,409,

' led August 9, 1949', and Iassigned vto the saine assignee as the present application.'

It has been experienced that the s urfacejof grains'or charges' compounded of an alkyl or polyester'type resin as' a binder, has sometimes'not bonded satisfactorily to the liners.- This has been duev to the difficulty in producingan adherence of the 4liner material to the surface of a cured alkyd or polyester resin base charge.

While some liners appear to bond fairly well and to` adhere to grains o-f the above-mentioned composition at ambient temperatures, sometimes such liners have separated from'the grain `when'subjected to higher or lower temperatures; Thisv is partly due to the factthat the physical properties `ofthe'linerV materials usually' vary considerably from those rof; the resin used to bind the propellant charge. When chilledV some'Y liners contract more than' the resin in the' charge an'd it thelliner-m-aterial becomesbrittle' at'low temperatures, it mayc'rack away fromthe charge, leaving.expos'edsur-fa'c'esA for burning; When'be'ate'd, some liners may expand considerably more than'the resin in the` charge, causing such/liner to .p

as ammonium or potassium perchlorates; however, other A 2,977,884 Patented Aprea, roei ous 'laminate which comprises a plurality of layers ofV fabric, such as glass cloth, metal cloth, fiber cloth and the like impregnated with a resinous material comprising a polyester resin with which there is incorporated a monomeric oleiinic component suchas a vinyl compoundor other oleiinic compound capable of cross-linking with the resin and compatible with the 'liquid resin; the polyester being the reaction-product of a polycarboxylic acidv with a polyhydric alcohol.

A related feature isr the coating of the surface of the grain itself with a somewhat similar but preferably harder polyester resin than-that used to form the laminate. This resin is applied tothe surface ofthe grain land serves to cement the laminate to the grain `vwhen the resin be-l tween the surface of the grain and the surface of the laminate'has been cured.

The foregoing and other featuresy of the invention Will be -better understood from the following detailed description and the accompanying draw-ing of which:y

Fig. 1 shows a cross section view taken through a propellant grain which is lined in accordance with this invention;

Fig. 2` shows a broken view ofv the surfaceof the i propellant grain of Fig. 1 protected by additional heatresistingy layers; and

Fig. 3 is an enlarged section of the laminate 12.

Polyester resins suitable as binder resins for propellant grains are set forth inthe said Roberts application Serial No. 109,409. One which is particularly suitable for the formation of a propellant `grain or chargecan be compounded by reacting 4 moles of sebacic acid, 5 moles of propylene glycol and 1 mole of maleic anhydride. Such a resin will hereafter be referred to as Resin A. The addition of styrene -to the condensation product obtained by reacting the above compounds will cause heteropolymerization of the polyester resin and permit the resin to become thermosetting after rheteropo-lymerization; and

the amount ofy styrene added to the resin determines'the degree' of flexibility of the finished product.

this-general type`,`useful -forl making grains are Aas fol-` lows:

adip'ic acid, 3 molesrof maleic anhydride and ll moles of ethylene glycol. This resinwill lhereafter be yreferred to as Resin'B. Y

A condensation product yformed by reacting 7 moles of adipic acid; 3vr moles of maleicanhydride and 12' moles of diethylene glycol. This resinwillbe referred to l1ere` after' as Resin C.

A Vcondensationproduct formed by. reacting 1 mole" diethylene glycol, 0.9 mo'le'of adipic ac'idand 0.1 ino'le maleic anhydride. after as Resin D.

A` condensation product This resinwill be referred to herevformed* by react-ing.- 1 moleV polypropylene glycol 425 (m:w.'), 0.9 mole adipic acid? Thisresinwill b'e'rfef l and 0.1k mole maleicanhydride. ferred to hereafter as `Resin E.

A suitable catalyst, such as benzoyl peroxide, tertiary butyl hy v A condensation pro-duct made by reacting 7 molesfof A condensation product formed by l mole polypropylene glycol 425 (m.w.), 0.8 mole adipic acid and 0.2 mole maleic anhydride. This resin will hereafter be rerefred to as Resin F.

A condensation product formed by reacting l mole polyethylene glycol 300 (m.w.), 0.9 mole adipic acid, and 0.1 mole maleic anhydride. This resin will hereafter be designated as Resin G.

A condensation product formed by reacting 1 mole polyethylene glycol 300 (m.w.), 0.8 mole adipic acid, and 0.2 mole maleic anhydride. This resin will hereafter be referred to as Resin H.

Examples of the vinyl or other olenic monomers capable of copolymerizing with the polyester resin are: vinyl compounds such as styrene, esters of acrylic and methacrylic acid such as methylacrylate and methyl methacrylate, and derivatives of vinyl alcohol, such as vinylacetate, acrylonitrile, etc. Othenexamples of compounds capable of cross-linking with the polyesters are diethylene glycol bis-(allyl carbonate), diallyl phthalate, diallyl maleate, diallyl diglycollate, diacrylate of esters of diglycol v such as diethylene glycol diacrylate, ethylene glycol acrylate, propylene glycoldiacrylate, butandiol1,3-diacrylate, etc.

In order to form a propellant charge a suitable oxidizer is added to the polyester resin-olenic copolymerizing monomer, capable of cross-linking with the polyester resin, by stirring and mixing. It is preferable to add the oxidizer to the mixture when the resin is still in the liquid stage. Examples of suitable oxidizers are inorganic compounds including the chromates, dichromates, perchlorates of metal such as the alkali metals, namely sodium, lithium, potassium, rubidium, and caesium, and inorganic compounds of the above anions in combination with nonmetallic radicals such as the ammonium radical and hydrazine. The selection of the oxidizer material depends upon the type of propellant and the specific burning properties desired. The preferred oxidizers are the perchlorates, especially the perchlorates of potassium and ammonium. The amount of oxidizer added to the resinous mixture usually is between 45%-90% by weight of the total propellant composition and the weight of the polyester resin-olenic cross-linking compound mixture should lie between 55%-l0% by weight of the propellant.

Catalytic substances are particularly useful for speeding up the rate of polymerization of the said polyester resin monomer mixtures with the oxidizer added. Such catalytic substances are the organic peroxides and the organic peresters. The temperature used for curing is dependent somewhat upon the nature of the catalyst and the time during which it is desired to accomplish complete polymerization.

The organic peroxides or peresters should be preferably soluble or compatible with the polyester resin. However, in some instances even an insoluble organic peroxide or perester'may function as a catalyst as long as it can be made to decompose and liberate a free oxygen radical. Specific examples of compounds which are suitable` catalysts for this polymerization reaction are: tertiary-butyl hydroperoxide, cumene hydroperoxide, benzoyl peroxide, lauryl peroxide, acetobenzoyl peroxide, ditertiary butyl peroxide, methyl ethyl ketone peroxide, l-hydroxycyclohexyl hydroperoxide-1, cycloalkane hydrocarbon peroxides and other hydro peroxides which are not too volatile at the curing temperature.

Some catalysts, such as l-hydroxycyclohexyl hydroperoxide-1, cumene hydroperoxide, are capable of polymerizing certain resins such as Resin A at room temperature provided that the resin and the catalyst are left to cure fora suicient period of time.

Specific examples of suitable peresters are: tertiarybutyl perbenzoate, and ditertiary-butyl diperphthalate.

Such catalyst should be present in the polyester resinvinyl or other suitable monomer mixture during the time it is subjected to the curing process. In general, the weight of the catalyst employed to bring about this result is approximately 0.5% by weight based on the weight of the combined polyester resin-vinyl or other suitable monomer mixture. If desired, larger amounts of the catalysts may be employed than those indicated.

In order to provide steady burning at low pressure with some oxidizers, it is beneficial to incorporate into the propellant mass approximately 1% by weight of carbon black which is added to the liquid mixture at the time the oxidizer is incorporated therein and before the mixture is cured.

The manner in which I prefer to compound the propellant is as follows: The amount of oxidizer required to cause the fuel to oxidize to the proper degree is uniformly stirred into the liquid polyester resin-vinyl, or other suitable monomer mixture. Stirring is continued at room temperature until all of the oxidizer has been added and the mixture has a uniform consistency. A catalyst capable of accelerating polymerizationv of the resin mixture is added thereto and intimately incorporated therein before or at the time the oxidizer is added. The stirred mixture, with the catalyst incorporated therein, is cast into a suitable mold, ordinarily cylindrical in outline, and the material is allowed to cure. The cast mass is generally cured at temperatures ranging from ambient to 220 F. If lower temperatures are used, the charge may require a considerable period of time to cure. If a shorter time is desired, more elevated temperatures may be used.

The foregoing description of the manner of making the grain is no part of the present invention and is given in order to afford a good understanding of the type of grain with which the present invention is used.

Referring now to the drawing, there is shown a cross section View taken through a cylindrical-shaped propellant grain or.charge 10 of the polyester resin-oxidizer type. In accordance with this invention there is provided on the grain a coating 11 which is also a polyester resin base material and over this coating there is applied the fabric laminate 12 which is also made by employing a polyester resin similar to the one used in the charge as is described hereafter in detail. The exposed surface of the laminate 12 may be coated with a layer of thermoplastic cement 14 to which there is subsequently applied a layer 15 of heat insulating material such as asbestos. This protective layer insulates the grain against the action of the hot gases and flames and may be cemented to the fabric laminate after vthe resin, which binds the resin to the grain, has been cured. Any suitable adhesive substance may be used to attach the asbestos or other insulating sheet to the laminate. A satisfactory form of adhesive for the above purv, pose is a thermoplastic cement obtained by dissolving cyclonized synthetic rubber in methyl ethyl ketone. This product is available under the trade-name of Pliobond 30 Cement.

The laminate 12, as stated above, comprises a plurality of layers of fabric material bound together by a polyester resin in which there has been incorporated a vinyl or other olefinic monomer capable of cross-linking with the polyester resin and can be made as follows: A polyester resin, for example, a resin similar to Resin A, formed by reacting a polyhydric alcohol with an unsaturated polycarboxylic acid and one or more saturated polycarboxylic acids, is mixed with a vinyl type of monomer or other olenic monomers capable of cross-linking with the condensation product, such as styrene, methylacrylate, methyl methacrylate or any of the other olefinic compounds listed above. The fabric layers which may be a glass, metal or liber cloth are impregnated with the unpolymerized resin-vinyl monomer mixture. These impregnated fabric sheets are positioned one on top of the other and compressed together, forming a single, thick laminated sheet. The pressed laminate is then cured, fusing all the fabric layers into a single laminate. These laminates, made of any suitable thickness, are strong yet ilexible, provided'the sheets arenotw made too thick;.rand when the fabric employed is glass cloth, andthelaminate has the advantage lthat it is fairly transparent. The cured laminate may now be applied to those surfaces of a propellant charge upon which no burning is desired, and

cemented thereto with a compatible resin. Although the y above described for use in making the grain-itself. However, anyy ofthe other polyester resins, such as'those described above for forming the propellant charge may be substituted in place of Resin A.

Thecoating 11 on the surface of the grain performs the function of cementing the fabric laminate intimately tothe grain afterboth the grain and the laminate have been cured. This coating 1l is also a form of polyester resin and a particularly useful form of such resin for this purpose is obtained by mixing the condensation product resulting from the reaction of a` polyhydric alcohol or alcohols and an unsaturated polycarboxylic acids with a vinyl monomer or other olenic compounds capable of cross-linking with the resin. These vinyl monomers are the same as previously disclosed in forming` the propellant charge, the preferred substances being styrene-methyl acrylate, methyl methacrylate, etc.

The unsaturated polycarboxylic acids, which may be reacted to form this class of polyester resins are the aromaticpolycarboxylic acids such as phthalic acid, isophthalic, tercphthalic and naphthalic acids; unsaturated polycarboxylic acids such as fumaric or maleic acids,- etc.; and anhydrides of both the unsaturated acidsand the aromatic acids, such as maleic anhydride and phthalic an-V hydride. It is also possible to employ combinations of the unsaturated acids and anhydrides, if desired. t Examples of the polyhydric alcohols which may be employed in formingthe polyester resin of this type are the diethylene glycol, ethylene glycol, propylene glycol, triethylene glycol, propane-1,2-diol, trimethylene glycol, and butane-2,3diol. v Y

A specific example of a typical. resin, suitable for cementing the-cured laminate to-the'propellant grain, is one comprising the condensationproduct obtainedbyreacting 3 moles of diethylene glycol with 3.6 moles-ofy ethylene glycol, 2.0 moles of phthalic anhydride, `3.4

. moles of fumaric acid. This resin will hereafter bey referred ytoas-Resin X. I

ResinX is considerably harder than Resin'A, whichis usedl in` forming the laminate andV propellant, yand this harderv quality isdesired-inorder that it-may Well perform' its function of'tenaciously adhering to boththegr'ain surface'and of'y cementing itself to thelaminate. Y

PolyesterY resins similar;v to Resin X tend to polymerize on standing aty ambient conditions, due to the high' degree of unsaturation in the molecule. If they are`to be preserved for any length of vtime before use, these resins should be inhibited against polymerization, for example, by adding' toV the liquidpolyesterA resin a small percentage o'fa'ninhibitor such as hydroquinone or catechol. These substanceseifectively inhibit' polymerization until a strong polymerization' catalyst, such as the organic`peroxide,for example, benzoyl peroxide or a perester is added; Y

After-the application ofthe liquid-Resin VX with one of thel cross-linkingfagents described`- above-to the'sur'face of the grain, .the curedlaminate-V is-:applied over fthe* coating of-lquid Resin X; an'd lthegrainithus lined is"subjecte`d to @temperature cure f to'4 polymerize the highly unsaturated Resin-fLX-cross-linking agent mixture. When cured this resin. permanently. cement'sthe laminate tothe grain` surface.v

propellant grain is as follows: The portions of the polymerized propellant grain which are to be restricted against burning. are rst scrubbed clean with a volatile solvent such as acetone, ether, petroleum ether, etc., to remove any dust or perchlorate particles, or any foreign material adhering to the surface. Unless the propellant grain is Well cleaned the'reliability of the restriction is materially lowered. f

The fabric laminate made from glass cloth, metalcloth, or fiber cloth, which has been prepared by impregnating the individual layers ofthe fabric with a resin comprises a mixture of Resin-A and styrene, or other cross-linking; agent. The amount of styrene or other cross-linkingagent depends on the degree of cross-linking required; asuitable" amountof cross-linking agent being aboutv 50%'. Lowerr amounts of the cross-linking agent tend to make the re'- sulting laminate more exible. The layers are compressed together and cured.- After curingthe laminate is cutto the desired shape.- y

The cured clean surfaces of the lpropellant to be insulated by the laminate are then painted with Resin X to which has been added vinyl monomer, or other crosslinking agent, capable of copolymerizing withY the resin.'

A catalyst, such as benzoyl peroxide is -also incorporated Y in yResin X before itis applied. The amount of styrene added to the Resin'X is dependent upon the desiredl viscosity of the uncured resin and is generally between 20%l and 30% by Weight based on the Weight'ofthe styrene mixture. The contact surface of the laminate is also coated with a layer of the unpolymerized Resin X. The laminate is then cemented to the'cleaned surface or surfaces of the grain to which Resin X has been applied, all air bubbles pressed out, and the entire propellant charge and liner are subjected to curing'temperatures andmain' tained at those temperatures until the layerv of resin 'be-` tween the charge and the laminate has becomev cured. The time required for'thecuring process of the binder resin is usuallyshort since the layer of `resin presentbei tween the grain and laminate is fairly thin'.Y Approximately four hours are generally suicient if the charge and liner are placed in a curing oven which isheldbetween 210 F.220 F. After curing the fabric laminate is integrally bondedto the surface of the charge.

The restriction contains no oxidizer, therefore, it will burn at a much slower rate than the propellant, for this reason it restricts or inhibits burning. This is notl effectively accomplished unless the' restriction is intimately bondedto the propellant grain surface'in which case it will insulate the propellant surface from the burning' temperatures surrounding the propellant restriction." This prevents the insulated propellant surface from burning until all of the grainhas been consumed.Y

An advantage is had when the 'laminate is of gla'ssor material having a Wide mesh, as this makesl itr` possible to observe the surface of the propellant charge after the laminate has been cemented thereto, inasmuch as the cured resin Aitself is transparent'. This makes itv possibleto detect the presence of any surface imperfections which could expose to burning a greater area than is desired.-

The liner-according to ourv invention not only possesses satisfactory properties vin tension and in compression', but also serves as an insulator against the ilamesandv hot gases released during the combustioneof the charge `inthe chamber. This increasesthe reliability of--the propellant charge.

LInI combustion, a solid propellantcharge which comprises a' cured intimate' mixture of asolid, inorganic* oxidizing salt,v said inorganic 'oxidizing saltb'ei'ng v present l ester resin -consistingessentially of'thecondensatironprod The *manner in which the restriction is applied' to the saturated polyester resin, consisting of the condensation product of saturated polyhydric alcohol and polycarboxylic acid, and an unsaturated compound selected from the group consisting of lower alkenes, lower alkynes, phenyl substituted lower alkenes, lower alkyl dienes, lower alkenyl esters of lower alkanoic acids, lower alkylesters of lower alkenoic acids, lower alkenyl esters of lower alkenoic acids, allyl diglycol carbonate, diallyl diglycollate,

`and mixtures thereof; and thereafter polymerizing said adhesive mixture and said laminating mixture so as to bind said laminate to said propellant charge.

15. The method of claim 14 wherein a layer of lireproof material is bound to the outer surface of said laminate after the said polymerization step.

16. The method of claim 14 wherein said propellant grain is washed with a volatile solvent prior to coating the surface of said grain with said unpolymerized adhesive mixture.

ReferencesCited in the tile of this patent UNITED STATES PATENTS 1,809,984 Mains June 16, 1931 2,229,208 Holm et al. Jan. 21, 1941 2,429,688 Hoover Oct. 28, 1947 2,443,736 Kropa June 22, 1948 2,443,739 Kropa June 22, 1948 2,443,740 Kropa June 22, 1948 2,479,828 Geckler Aug. 23, 1949 2,485,294 Kropa Oct. 18, 1949 2,528,235 Loritsch Oct. 31, 1950 2,539,404 Crutcheld Jan. 30, 1951 

